Topic 6: Carbohydrates and Fertilisers
Carbohydrates are a group of foods, the main purpose of which is to provide
slow-release energy. Examples of high carb foods are potatoes, rice and wheat
products (like bread, cakes and pasta).
Carbohydrates are made up of 3 elements only.
Carbohydrates
Products of Combustion
Carbohydrate
e.g. sugar or
bread
Since carbohydrates contain
carbon and hydrogen, the
products of its combustion
are the same as that of a
hydrocarbon.
_____________ turns
limewater cloudy.
_______________ boils at
1000c, freezes at 00c and
turns cobalt chloride paper
from blue  pink
Carbohydrates can be classified as a Sugar or Starch.
Sugars
Sugars are sweet-tasting substances, of which there are hundreds. The 4
sugars we will deal with are Fructose, Glucose, Maltose and Sucrose.
Starch
Starch is a white, powdery solid which does not taste sweet.
Carbohydrates can also be classified as Mono-saccharides, di-saccharides
and poly-saccharides.
Mono-saccharides
Fructose
Glucose
C6H12O6
C6H12O6
Fructose is found in fruits & honey
Glucose is found in many sports drinks
Fructose & Glucose are ___________, as they have the same molecular
formula, but different structural formula.
Di-saccharides
Maltose
C12H22O11
Formed when 2 glucose molecules join up.
Sucrose
C12H22O11
Formed when glucose & fructose join up.
Lactose
C12H22O11
Found in milk.
Di-saccharides are made by combining 2 mono-saccharide molecules. We would
expect the di-saccharides to have the molecular formula, C12H24O12.
However, when the mono-saccharides join up, water (H2O) is removed. When 2
smaller molecules combine to form a larger molecule, with water being removed;
this is called a ____________________________ reaction.
Polysaccharide
Starch
(C6H10O5)n
Formed from many glucose molecules.
Starch is a huge molecule, formed from thousands ofglucose molecules, in a
condensation reaction. “n” is the no. of glucose molecules which combine.
Important Rule
A carbohydrate can be identified by its molecular formula, as there are
always twice as many hydrogen atoms as there are _____________ atoms.
Testing for Carbohydrates
Starch
Starch is the only carbohydrate which reacts with iodine solution. Iodine
solution turns from yellow to ________________ in the presence of starch.
Sugars
Sugars can be tested with _______________ solution. All sugars, with the
exception of Sucrose, turn Benedict’s from blue to _____________________.
Starch also has no effect on Benedict’s as it is not a sugar.
Sucrose & Starch Suck!!!
Digestion of Starch
Starch is the main carbohydrate in our diet. However, due to the large size of
its molecules, it is too big to pass through the walls of our stomach and into our
bloodstream. Therefore our bodies need to break the starch down into the
sugars which make it up. If starch is formed by joining up lots of glucose, by
removing water, then it can be broken down by adding the water back in.
This process of breaking up starch, using water is called ______________.
Hydrolysis is carried out by the enzyme, salivary amylase, in the mouth and by
acid in the stomach.
STARCH + “H2O”
Glu
Glu
Glu
Glu

Glu
+ “H2O”
GLUCOSE
Glu

Glu
Glu
Glu
Glu
Making Carbohydrates
Plants are essential to human and animal life, as they provide the carbohydrates
for our diet and also the ______________ that we breathe. Both are
produced in a reaction called ____________________, where plants use light
energy from the sun, to initiate a reaction between the __________________
it absorbs through the leaves and the ____________ it absorbs through its
roots. The green pigment ________________, found in plant leaves enables
the light energy to be harnessed.
Oxygen
Carbon
Dioxide
Carbohydrate
Using Carbohydrates
In humans and animals, carbohydrates are used to provide energy. This process
involves the carbohydrate reacting with ________________, to produce
__________________ and water, whilst releasing energy. This process is
called ________________. This process is the reverse of photosynthesis.
Carbohydrate
Carbon Dioxide
Oxygen
Water
Respiration
Carb + O2
CO2 + H2O
Dioxide+Wat
er
Photosynthesis
Using Carbohydrates: Making Ethanol
Ethanol is the alcohol that is contained in alcoholic drinks like beer and wine. It
is also used as a fuel.
Ethanol has been mixed with petrol for years, in South America.
Petrol sold in the USA is made up of 10% Ethanol and Britain is starting to do
the same.
Ethanol is considered by some to be a fuel of the future, as it can be produced
from sugar cane, thus is considered to be a ________________ fuel, since it
can be made over and over again. It is not finite, like crude oil.
The reaction used to make ethanol from sugar is called _________________.
This involves adding ____________ to sugar solution (this can be in the form
of mashed up fruits, vegetables or grains).
An enzyme in the yeast converts the plant sugars into ethanol, in the absence of
oxygen, O2.
_________________ gas is also produced in this reaction.
C6H12O6
Glucose solution


Yeast
C2H5OH + CO2
Ethanol + CO2
Ethanol molecules have this structure:
The structure is similar to an alkane. It has 2 carbons, so it has the “eth”
prefix.
The “C-OH” at the end of the molecule is called a Hydroxyl functional group. It
is this group which gives the “ol” ending to ethanol.
Ethanol belongs to a family of compounds called the Alkanols. Alkanols are
commonly called “alcOHols” because of the C-OH functional group
Increasing ethanol content
In the fermentation process, it is only possible to produce a maximum ethanol
concentration of 14%.
This occurs because ethanol is a poison.
At 14% concentration, the yeast is killed off and no more ethanol is produced.
This is far too low to be used as a fuel.
Also, alcoholic spirits such as vodka or whiskey commonly have alcohol
concentrations of 35-40%.
These higher concentrations are reached by separating the ethanol from the
water.
Ethanol has a boiling pt of 740C and water has a boiling pt of 1000C.
Liquids with different boiling points can be separated using the process
__________________.
This is similar to separating crude oil fractions.
Distillation is carried out to
produce higher ethanol
concentrations
 higher conc. alcohol solution
Plants: The source of all our food
Plants are essential for our survival as they are the source of all our food.
We eat plants and the animals we use for food, also get their nourishment from
plants.
In the year 18
00, the world’s population stood at 1 billion people.
In the year 2001, global population reached 6 billion.
Now, in 2009, the population has reached 7 billion.
The world’s population is predicted to rise to 10 billion by 2050.
Key Point
Due to this increase in
global population, it is
necessary to grow more
food crops, to feed
everyone.
Furthermore, biofuels (fuels made from plant products like sugar cane) are
becoming more popular, thus more plants must be grown to keep up with demand.
Nutrients for plants
Plants, like humans, require food to enable them to grow. Like humans, they also
require trace elements to keep them healthy (e.g. we require iron in our diet, to
enable us to transport oxygen around our bodies effectively).
There are 3 main elements that are essential for healthy plant growth.
_______________, N
_____________, P
_____________, K
Plants get these nutrients from the soil. They absorb the nutrients through
their ___________.
When these nutrients are used up, they must be replenished. Farmers and
horticulturalists add ________________ to soil, to replace the nutrients.
Fertilisers can be “Natural” e.g. manure or compost; or they can be “Synthetic”
(man-made).
Different plants require different ratios of nutrients; some require lots of
potassium but only a little phosphorus.
Thus, artificial fertilisers have an “NPK Ratio”, showing the proportion of each
element that they contain.
As plants absorb their nutrients through the soil, it is important that fertilisers
are ______________, so that they dissolve in groundwater and are taken up by
the plant’s roots.
However, this solubility can also be a
problem, as the fertiliser can be
washed into rivers and streams, if
there is heavy rain. This can pollute
the river water and cause depletion in
dissolved oxygen, thus fish and marine
life are killed off.
Extraordinary Plants
Even though the air is 79% nitrogen, most plants cannot use this as the nitrogen
is very unreactive, due to the triple covalent bond in the N2 molecule. However,
“leguminous” plants like _________, __________ and clover, have “nodules” in
the roots, which contain rhizobium bacteria that are able to convert the
atmospheric nitrogen into soluble nitrate compounds. These plants are called
“Nitrogen-Fixing” plants. Since these plants effectively make their own
fertiliser, then farmers can plow these back into their fields and other plants
can use the nutrients.
Making Fertilisers
The most common fertiliser is ammonium nitrate. Ammonium nitrate is a soluble
salt. Salts are formed from reactions between ___________ and _________.
In this case _________ acid is titrated with _________________ hydroxide.
This neutralisation reaction produces ammonium nitrate salt.
The titration is carried out, using universal indicator to show the end
point. The end point occurs when the indicator turns from red to
_____________.
The volume of alkali required is noted and the titration is repeated
without the indicator, to produce a pure solution of ammonium nitrate.
Water is then evaporated
from the solution, leaving
solid ammonium nitrate
Ammonia
Ammonium Hydroxide is produced by dissolving ammonia gas in water. Ammonia,
or Nitrogen Hydride is a very important chemical.
______(g)
Ammonia
+
H2O(l)
Water

NH4OH(aq)
Ammonium Hydroxide
Ammonia has the chemical formula, ______. It is a soluble gas which turns
moist pH paper _________. Therefore it is an alkaline gas.
It is used as a cleaning agent and also in hair dyes. It has a strong, “fishy” smell.
A pyramidal shaped Ammonia molecule
Making Ammonia: The Haber Process
The industrial process of making ammonia is called the Haber Process. The
reactants in this reaction are _________________ and ________________.
N2
+
H2
NH3
(balance the equation)
means that this is a reversible reaction, so it is possible for
the ammonia to break back down to nitrogen and hydrogen.
Any unreacted nitrogen or hydrogen, can be fed back into the reactor and
used again.
The
Temperature
Pressure
Catalyst
This reaction is very slow at room temperature, so we use a moderately
high temperature to speed up the reaction.
We also have the reactants at a high pressure to maximise the volume of
ammonia formed (we look at this in more detail in Higher Chemistry).
As a final measure to speed up the reaction, we use an iron catalyst.
Important- Frequent Exam Question
Ammonia can be produced in the lab, by heating an ammonium salt, with
an ____________. e.g.
NH4Cl
+
NaOH

Ammonium Chloride Sodium Hydroxide
NaCl + H2O + NH3
Ammonia
Using Ammonia: The Ostwald Process
In the chemical industry, ammonia is used to make nitric acid. This is a
very important chemical in the production of fertilisers, pharmaceuticals
and explosives. This is a 3-step reaction.
The ammonia is reacted with ___________and the reaction is carried
out using a _____________ catalyst.
4NH3 +
5O2

Pt Catalyst
4NO +
6H2O
The colourless nitrogen monoxide is then reacted with more oxygen to
form brown nitrogen dioxide.
2NO + O2 
2NO2
The nitrogen dioxide is then dissolved in water to form nitric acid
NO2 +
H 2O

HNO3
Nitric Acid
Important Point
In the Ostwald process, an initial temperature of 9000C is used.
However, the reaction mixture only needs to be heated at the start of
the reaction, as it is an _______________ reaction, which gives out
heat. Since the reaction produces its own heat, it is not necessary to
continue heating once the reaction is under way.
% Composition
In order to establish the NPK ratio of a fertiliser, it is essential for the
manufacturer to calculate what percentage of the mass, each element
makes up.
e.g. In a class of 20 pupils, there are 10 girls. To find out what
percentage of the class, girls make up:
No. of Girls
No. of pupils
X 100 (to convert to %)
Therefore 10/20 = 0.5 and 0.5 x 100 = 50%
For a chemical compound, we use the same principle.
To calculate the % of mass, that nitrogen makes up in ammonia NH3
RAM of element x No. of atoms of the element
Formula mass of compound
RAM of nitrogen = 7
X 100
No. of N atoms in NH3 = 1
FM of NH3 = 7 + (3x1) = 10
Therefore (7x1)/10 = 0.7 and 0.7 x 100 = 70%
Nitrogen makes up 70% of the mass of Ammonia
Calculate the % of mass that potassium makes up, in potassium phosphate
K3PO4
Empirical Formula
Imagine we have a compound, which we do not know the molecular formula
of. If we know the % of mass that each element makes up, then we can
work out what its Empirical Formula (or simplest molecular formula) would
be.
e.g. If we have a hydrocarbon compound, where carbon makes up 75% of
the mass and Hydrogen makes up 25% of the mass.
Step 1: Convert % to decimals.
75% /100 = 0.75
25%/100 = 0.25
So in 1g of the hydrocarbon, C makes up 0.75g and H makes up 0.25g
Step 2: Work out no. of moles of each element
Moles =
Mass
RAM
RAM of C = 12
RAM of H = 1
So for 1g of hydrocarbon
Moles of C= 0.75/12
= 0.0625 moles
Moles of H = 0.25/1
= 0.25 moles
Step 3: Divide no. of moles of each element by the smallest no. of moles
By doing this we can work out the no. of atoms of each element in the compound
The smallest no. of moles in this instance is the 0.0625 moles of carbon.
For C; 0.0625/0.0625 = 1, Therefore there is 1 atom of Carbon
For H; 0.25/0.0625 = 4, Therefore there are 4 atoms of Hydrogen.
Therefore the empirical formula of the hydrocarbon is CH4.